truck wont shift

I have a 1991 Chevy k1500 4x4 pick up. When I tried to drive it, it wouldn't shift out of first gear (it's an auto). I'm not very mechanically smart, so don't know a whole lot about cars, but I am smart enough to have checked tranny fluid and it is full. What could be causing this?? I really don't want to replace tranny as it would probably be too expensive. Is there any easy fix to this? PLEASE HELP ASAP!@@ This is my daily driver and I don't have another vehicle to drive

Check the governor and TV cable hook up and adjustment.

Is low and reverse good and solid or slipping?

If good pull governor and check gear and for weights being stick.

Let us know how it goes.